A portable battery with high performance is one of key elements used in producing finished products such as various mobile information communication instruments, electronic devices, and electric vehicles. The next generation type energy storing systems that have been developed in recent years have been making good use of the electrochemical principle; e.g., a Li-based secondary battery and an electrochemical capacitor.
The secondary battery is an efficient energy storage device in part because of the large amount of energy stored per unit weight or unit volume. However, development of secondary batteries continues in order to improve on run time, charge time, and the amount of energy stored per unit time (power density).
The energy density of the electrochemical capacitor is relatively small compared to that of the secondary battery, but the electrochemical capacitor offers superior running time, charging time, and power density compared to current secondary battery technology. Therefore, many studies have been conducted on electrochemical capacitors to provide increased energy density.
A super capacity electrochemical capacitor manufactured using an electrochemical principle is divided into two groups: an electrical double layer capacitor (EDLC) which utilizes the principle of an electrical double layer, and a supercapacitor capable of generating super capacity 10 times greater than that of the EDLC type due to pseudo-capacitance generated in an electrochemical faradaic reaction.
In EDLC, an active carbon fiber is used as an electrode active material of a capacitor, thus storing a high intensive electric charge in an electrical double layer, while the supercapacitor uses a metal oxide as an electrode active material. With the above high capacity and high power density, the supercapacitor may be well suited for the power of an electric vehicle, the power of a portable mobile communication instrument, the power of a memory backup of a computer, the power of a military/aerospace equipment, and the power of a small sized medical equipment.
The electrode material of the metal oxide can be considered the most important element in the supercapacitor formed of a metal oxide electrode, a separator, an electrolyte, a current collector and casing, and terminals.
Examples of metal oxide electrode materials that are suitable for use in supercapacitor applications include RuO2, IrO2, NiOx, MnO2, and the like.
Among the electrode materials discussed supra, RuO2 is known to have the highest capacitance (720 F/g), but its use is very limited to aerospace, aircraft, military, and related industries, due in part to high materials cost. Certain other materials which have previously been investigated as a RuO2 replacement, such as NiOx, CoOx, MnO2, and the like, have a very low capacitance and are therefore not particularly suitable for use as a high capacity metal oxide in electrode applications.
It would be desirable to develop alternate, less expensive electrical materials having capacitances that are similar to or greater than the capacitance of RuO2. It would also be desirable to provide electrodes and devices incorporating the use of said alternate electrical materials.
U.S. Pat. No. 6,129,901 discloses a method of producing metals containing carbon nanotube comprising: anodizing an aluminum substrate in an electrolytic cell for manufacturing an aluminum template having a plurality of pores, depositing an effective catalyst (selected from Co, Fe, Ni, alloys thereof) into the pores, and exposing the alumina template into a hydrocarbon gas at a temperature of 600˜800° C. so that carbon nanotube is pyrolyzed in the pores. Here, the outer diameter of each carbon nanotube is smaller than the diameter of the pore in the templates.
In the method for manufacturing recited in the '901 patent, metal oxide powder fabricated by a sol-gel method and a chemical precipitation method is mixed with a conductive carbon, and the mixture is formed in a predetermined shape according to the current collector, thereby forming an electrode.
The preparation of the metal oxide powder is synthesized using the sol-gel method, a multi-step process comprising agitating, filtering, washing, drying, heat treatment, etc. is needed, thus requiring a long period of time for mixing metal oxide materials.
Further, in order to form a shape of an electrode using thus synthesized metal oxide powder, a certain amount of conductive carbon and binder should be mixed. Therefore, it is impossible to accurately judge electrochemical properties of a metal oxide, and a specific capacitance is decreased due to the presence of a binder and a conductive material in addition to an active material.
Thus, for at least the foregoing reasons, it would be desirable to provide a low-cost reproducible high-specific capacitance material for use in electrochemical capacitor applications. It would also be desirable to provide a method for making nano-structured electrodes of the low-cost reproducible high-specific capacitance material which are suitable for use in electrochemical capacitor applications.